DevTools: Fix some missing images after switching to svg

select elements were incorrectly relying on chromeSelect.svg, which
doesn't exist yet.

Simimlarly, code folding was relying on the png version of treeoutline
triangles.

Change-Id: Id2f27a1800a5070e5eaa607260328f14cb06c582
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1663322Reviewed-by: Erik Luo <luoe@chromium.org>
Commit-Queue: Joel Einbinder <einbinder@chromium.org>
Cr-Commit-Position: refs/heads/master@{#669981}

third_party/blink/renderer/devtools/front_end/text_editor/cmdevtools.css

@@ -612,7 +612,7 @@ div.CodeMirror:focus-within span.CodeMirror-nonmatchingbracket {
 .CodeMirror-foldgutter-folded::before {
     background-color: #727272;
     -webkit-user-select: none;
-    -webkit-mask-image: url(Images/treeoutlineTriangles.png);
+    -webkit-mask-image: url(Images/treeoutlineTriangles.svg);
     -webkit-mask-size: 32px 24px;
     content: "";
     display:block;

third_party/blink/renderer/devtools/front_end/ui/inspectorCommon.css

@@ -240,7 +240,7 @@ input {
     outline: none;
     padding-right: 20px;
     padding-left: 6px;
-    background-image: url(Images/chromeSelect.svg);
+    background-image: -webkit-image-set(url(Images/chromeSelect.png) 1x, url(Images/chromeSelect_2x.png) 2x);
     background-color: hsl(0, 0%, 98%);
     background-position: right center;
     background-repeat: no-repeat;